1. Field of the Invention
The present invention relates to the bleaching of plant materials, more particularly plant materials destined for nutritional consumption, by means of hydrogen peroxide in an aqueous alkaline solution to enhance the whiteness thereof.
The plant materials which can be bleached according to the invention include all products of vegetable origin, which are used for nutrition, either in their entirety or parts thereof. Exemplary such products are cereal grains (wheat, maize, oats, barley, rice, etc.), peels, skins, pips of fruits, bran from oil plants, such as sunflowers, bran from cereals; it is also envisaged to bleach residues of various products such as beet or sugar beet, fruits such as pears, peaches, apples, apricots, grapes, oil plants, such as sunflowers, soya bean, citrus fruit such as lemon, cereals, residues resulting from extracting the fraction of these products which normally enhances their value, namely, sugar, fruit juice, pectin, oil or starch. It is also envisaged to bleach residues of fruit and cereals after alcoholic fermentation (grape malt, residues from alcoholic distillation, etc.) or almond shells, or shells of nuts with or without the fruit.
2. Description of the Prior Art
Plant materials of the above type are typically treated with aqueous alkaline solutions which contain hydrogen peroxide to "delignify" and bleach them.
Such bleached plant materials have a high plant fiber content and can be used, especially, for producing edible food products and, in particular, low-calorie food products, for example dietary flours.
However, the food products obtained from these fibers are too dark in color to permit them to be used as flour substitutes.
The known processes for converting plant materials into products which have a high cellulosic vegetable fiber content and which, moreover, have a whiteness suitable for commercial use, are generally processes carried out in aqueous medium. Thus, U.S. Pat. No. 4,649,113 to Gould describes the treatment of various vegetable fibers with H.sub.2 O.sub.2 in an alkaline medium. The dietary fibers are suspended in water in amounts ranging from 20 to 300 g/l in such manner that they are completely wet, then H.sub.2 O.sub.2 and NaOH are added, and the action of H.sub.2 O.sub.2 occurs in an aqueous medium.
FR-A-2,647,641 and EP-A-337,653, for example, describe treating the plant materials prior to the bleaching step with an acid or basic solution, such as to improve, notably, the whiteness thereof. Each of these processes is carried out in an aqueous medium.
These bleaching processes present disadvantages because it is necessary for them to be carried out in an aqueous medium. Thus, the cereal-derived dietary fibers, such as wheat bran, maize bran, etc., always contain a substantial amount of flour or starch and, when wetted, form very viscous "pastes" which are very difficult to stir and to manipulate. The bleaching medium must therefore be highly diluted.
Moreover, the dietary fibers which have a very high water retention capacity, such as, for example, beet pulp, increase up to 10-fold in volume and weight. This reduces the productivity of the processing apparatus.
Finally, the diffusion of the reactants into the dietary fibers requires a lengthy period of time and necessitates either high-performance and very expensive stirrers/mixers, or long reaction times in a highly diluted medium with traditional stirring.
FR-A-2,651,965 describes a process for bleaching vegetable fiber in which the vegetable fiber, which has a consistency of greater than or equivalent to 10% (namely, a moisture content &lt;90%), preferably ranging from 20% to 35% (namely, a moisture content ranging from 65% to 80%), is soaked with an alkaline aqueous hydrogen peroxide solution and bleached during drying.
This process enables a bleached dry fiber to be obtained whose degree of whiteness is superior to the traditional processes in a liquid medium, but which, on the other hand, present another shortcoming: the bleaching and dried fiber contains a variable amount of residual hydrogen peroxide which originates from the incomplete reaction of H.sub.2 O.sub.2 during the drying step.